The present disclosure generally relates to acidizing a subterranean formation, and, more specifically, to methods for acidizing a subterranean formation in the presence of ferric iron.
Treatment fluids can be used in a variety of subterranean treatment operations. Such treatment operations can include, without limitation, drilling operations, stimulation operations, production operations, remediation operations, sand control treatments, and the like. As used herein, the terms “treat,” “treatment,” “treating,” and grammatical equivalents thereof refer to any subterranean operation that uses a fluid in conjunction with achieving a desired function and/or for a desired purpose. Use of these terms does not imply any particular action by the treatment fluid or a component thereof, unless otherwise specified herein. More specific examples of illustrative treatment operations can include drilling operations, fracturing operations, gravel packing operations, acidizing operations, scale dissolution and removal operations, sand control operations, consolidation operations, and the like.
Acidizing operations may be used to stimulate a subterranean formation to increase production of a hydrocarbon resource therefrom. Introduction of an acidizing fluid to the subterranean formation may take place at matrix flow rates without fracturing of the formation matrix, or at higher injection rates and pressures to fracture the formation (i.e., an acid-fracturing operation). During an acidizing operation, an acid-soluble material in the subterranean formation can be dissolved by one or more acids to expand existing flow pathways in the subterranean formation, to create new flow pathways in the subterranean formation, and/or to remove acid-soluble precipitation damage or scale in the subterranean formation. The acid-soluble material being dissolved by the acid(s) can be part of or formed from the native formation matrix or can have been deliberately introduced into the subterranean formation in conjunction with performing a treatment operation (e.g., proppant or gravel particulates). Illustrative substances within the native formation matrix that may be dissolved by an acid include, but are not limited to, carbonates, oxides and aluminosilicates. Other substances can also be dissolved during the course of performing an acidizing operation, and the foregoing substances should not be considered to limit the scope of substances that may undergo dissolution by an acid.
Carbonate formations can contain minerals that comprise a carbonate anion (e.g., calcite (calcium carbonate), dolomite (calcium magnesium carbonate), and siderite (iron carbonate)). When acidizing a carbonate formation, the acidity of the treatment fluid alone can be sufficient to solubilize the carbonate material by decomposing the carbonate anion to carbon dioxide and water, thereby leeching a metal ion into the treatment fluid. Both mineral acids (e.g., hydrochloric acid) and organic acids (e.g., acetic and formic acids) can be used to treat a carbonate formation, often with similar degrees of success. Since it is relatively inexpensive, hydrochloric acid is very commonly used, typically in concentrations up to about 28% by volume.
Siliceous formations can include minerals such as, for example, zeolites, clays, and feldspars. As used herein, the term “siliceous” refers to a substance having the characteristics of silica, including silicates and/or aluminosilicates. Dissolution of siliceous materials during an acidizing operation is thought to be considerably different than acidizing carbonate materials, since the mineral and organic acids that can be effective for acidizing carbonate materials may have little effect on siliceous materials. In contrast, hydrofluoric acid, another mineral acid, can react very readily with siliceous materials to promote their dissolution. Oftentimes, a mineral acid or an organic acid can be used in conjunction with hydrofluoric acid to maintain a low pH state as the hydrofluoric acid becomes spent during the dissolution of a siliceous material.
When high concentrations of an acid, particularly hydrochloric acid, are present during an acidizing operation, the presence of ferric iron can be an extreme concern for reasons that are discussed hereinbelow. Ferric iron may be derived from a number of sources during an acidizing operation. In some cases, ferric iron can be transported to a subterranean formation via corrosion or descaling of tubulars and storage vessels through which an acid is passing. In some instances, this can be an unfortunate consequence in certain geographic regions where equipment may be poorly maintained or improperly used. In other cases, ferric iron may be solubilized in a subterranean formation through dissolution of an iron-containing mineral therein (e.g., iron carbonate, goethite, magnetite, hematite, and the like). Due to its ready oxidation, any initially produced ferrous iron is usually oxidized to produce ferric iron.
The presence of ferric iron during an acidizing operation can be problematic for a number of reasons. Foremost, as the acid becomes spent during dissolution of the matrix and the pH of the treatment fluid rises, gelatinous ferric hydroxide, can precipitate from the initially solubilized ferric ions. Such precipitation can begin to occur at a pH of about 2 and be essentially complete at just above a pH of about 3. In addition, ferric iron can readily form insoluble materials with sludge-forming components such as, for example, asphaltenes, maltenes, and porphyrins or related macrocyclic compounds, any of which may constitute a component of many crude oils. Sludging can also be exacerbated by the acidic nature of the treatment fluid. Hydrochloric acid, especially at concentrations of about 15% by volume or above, can particularly promote sludge formation in the presence of a sludge-forming component. Sludges, including ferric hydroxide, and other insoluble materials formed from ferric iron can be exceedingly detrimental for production of a hydrocarbon resource, since the very acidizing operation that was meant to increase the permeability of a subterranean formation may instead detrimentally decrease it.
A number of approaches have been developed for addressing the presence of ferric iron in a subterranean formation. Ferrous hydroxide is much more soluble than is ferric hydroxide, the former remaining soluble up to a pH of about 7. Therefore, reducing agents and/or antioxidants have been employed to reduce ferric iron to the ferrous state and/or maintain it there. Ascorbic acid, erythorbic acid, and related reducing agents have often been used in this regard. Another strategy that has been used for iron control involves the sequestration of ferric iron with a chelating agent. Still another strategy involves interacting the ferric iron with a hydroxycarboxylic acid to suppress its precipitation. However, all of these approaches may have difficulties. Chelating agents and other additives may be expensive and not readily soluble in the highly acidic fluids that are often used to carry out an acidizing operation. Solubility may be particularly compromised at hydrochloric acid concentrations of about 15% by volume or above. In addition, chelating agents sometimes may not be effective for forming a metal complex at low pH values, and/or they may not effectively form a metal complex at higher temperatures. Similarly, it can be difficult to maintain the highly oxidizable ferrous ions in a reduced state. Moreover, some of the reducing agents that can effectively maintain iron ions in their reduced state can have detrimental impacts on a subterranean formation.